SPC AC 041955

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0255 PM CDT Tue Aug 04 2020

   Valid 042000Z - 051200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   INTO TONIGHT ACROSS MUCH OF NEW ENGLAND AND PARTS OF THE CENTRAL
   GREAT PLAINS...

   ...SUMMARY...
   A risk for tornadoes will continue in association with Isaias,
   across southern New England by late this afternoon, and perhaps
   through northern New England this evening.  Otherwise, scattered 
   thunderstorms across parts of northern and central Great Plains may
   pose a risk for severe hail and wind late this afternoon and
   evening.

   ...Discussion...
   The only notable change is to reduce hazard probabilities across
   southern New England in association with the northward movement of
   Isaias.

   ..Smith.. 08/04/2020

   .PREV DISCUSSION... /ISSUED 1108 AM CDT Tue Aug 04 2020/

   ...Synopsis...
   Large-scale mid-level troughing remains amplified across much of the
   Mississippi Valley into the Appalachians, but as a significant
   embedded short wave impulse pivots across and northeast of the Great
   Lakes region, some loss in amplitude appears likely to commence
   later today and tonight.  This perturbation is preceded by Isaias,
   now accelerating north-northeastward across the northern Mid
   Atlantic coast, and likely to gradually become absorbed within the
   larger-scale cyclonic flow developing across the St. Lawrence Valley
   by late tonight.

   Upstream, some suppression of mid-level ridging appears probable
   across the Great Plains, perhaps more notably across the northern
   portions, than central and southern, associated with a more
   substantive digging short wave impulse now near the international
   border.  

   Given the persistence of the eastern troughing, seasonably high
   moisture content air is now largely confined to near and east of the
   Atlantic Seaboard, with steep lower/mid tropospheric lapse rates
   largely confined to the Intermountain West, Rockies and adjacent
   portions of the Great Plains.

   ...Northeast...
   Clockwise-curved low-level hodographs remain maximized near the core
   of the strongest 850 mb flow curving to the east and north of
   Isaias, along which there has been a persistent arcing band of
   convection, with embedded low-level mesocyclones.  This band appears
   likely to pivot north of Long Island through much of southern New
   England by late afternoon, accompanied by a continuing risk of
   tornadoes, as lower/mid 70s surface dew points advect ahead of it. 
   Models suggest that this low-level moist inflow will eventually
   become cut off, which may lead to decreasing potential for tornadic
   development across northern New England this evening.

   ...Front Range into central Great Plains...
   Although late afternoon mid-level height falls may be more subtle
   than farther north, models suggest that mixed-layer CAPE will become
   maximized on the order of 2000 J/kg, to the lee of the Front Range
   by late this afternoon.  Aided by orographic forcing and perhaps
   lift associated with a subtle mid-level impulse, thunderstorms are
   expected to initiate off the Front Range, then advect off the higher
   terrain within modest northwesterly mid-level flow.  Aided by strong
   shear, isolated supercells and organizing storm clusters are
   possible, which may pose a risk for severe hail and strong wind
   gusts into this evening.

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